The purpose of any four-way hydraulic directional control valve is to direct the flow and pressure from a uni-directional input device, such as a pump, to a bi-directional output device, such as a cylinder, to cause it to alternately extend and retract, thus moving or holding an external load in either or both directions of travel. At the same time, the valve directs the return flow of hydraulic fluid from the unpressurized end of the cylinder to the hydraulic reservoir.
Typical valve designs include balanced spool, shear-seal slide, and poppet. The balanced spool design is one in which a hydraulically balanced spool with undercuts is positioned axially within a fixed sleeve with internal under-cuts to provide the desired flow patterns. A major advantage is that only a very low force is required to move the spool since it is pressure balanced. A major disadvantage is that, due to necessary spool-to-sleeve clearance, inter-port leakage is unacceptable in high pressure holding applications.
The shear-seal slide design has a flat faced slide block with internal porting positioned axially along the top of four inline pressure loaded shoes (P,T,A,B). A major advantage of this design is that there is virtually no inter-port leakage due to pressure loaded shoes in contact with the face of the slide, both of which are lapped flat. Roller bearings on top of the slide prevent the slide from lifting while minimizing axial slide force. A major disadvantage is that a large force is required to position the slider due to friction between the shoes and the slide block.
The poppet design has multiple poppets (balls) that are urged on or off their seats to block flow or allow flow past their seats. A major advantage is that there is virtually no inter-port leakage due to the design of the poppet on the seat. A major disadvantage is that existing designs are unnecessarily complex and therefore costly.
Common methods of valve actuation include mechanical (manual knob, lever, cam follower, etc.), remote air or hydraulic pilot pressure, and electric solenoid. A solenoid is an electromechanical device which converts electric power into linear mechanical force and motion. Its counterpart in a hydraulic system is a cylinder.
Standard valve input/output port identification is used. The "P" port supplies pressure from the pump. The "T" port handles the return flow from the valve to the tank or hydraulic reservoir. The "A" port is the cylinder port to either the "head-end" or the "rod-end" of the hydraulic cylinder, while the "B" port is the cylinder port to the other end of the cylinder.
Examples of the prior art include U.S. Pat. No. 3,736,958, Four Way Solenoid Selector Valve (Rostad et al.). This is a 3 position, spring-centered, balanced spool (high leakage) design pilot-operated by two 3-way poppet solenoid valves using control orifices and metering lands for controlled (slow, non-shock) shifting of the spool. It is not a zero leakage valve. It is not fail safe for either loss of electrical power to either solenoid or loss of "P-port" pressure.
U.S. Pat. No. 4,574,844, Four Way Poppet Valve (Neff and Fagerlie) discloses a 2 position, direct-acting (as opposed to pilot operated), spring offset, balanced spool with fixed elastomeric poppets at each spool land face to provide a zero leakage 4-way, single solenoid operated valve. It is a low pressure pneumatic valve, unsuitable for high pressure operation because the elastomeric poppets would extrude into the gap between the spool and the housing and eventually blow out or get sheared off. Further, the distance between the poppet faces and the seat faces need only to be approximately equal because compression of the poppets would make up for small differences and still maintain zero-leakage. If the poppets were metal (for high pressure sealing), the gaps would have to be made and maintained exactly equal. It is not fail safe for either loss of electrical power to either solenoid or loss of "P-port" pressure.
U.S. Pat. No. 4,842,020, Double Solenoid Single-Stem Four-Way Valve, (Tinholt) discloses a 2 position, direct-acting, double solenoid, unbalanced spool design with a fixed elastomeric poppet mid-span on the spool (seals "pressure" to the selected "load port") and elastic 2 "load ports to O-rings near each end of the spool. It provides a zero leakage 4-way double solenoid valve. exhaust ports". It is a low pressure pneumatic valve, unsuitable for high pressure operation for the same reasons as the Neff patent described above. Additionally, the force required to shift the spool would be unacceptable at high pressure due to both the unbalanced force and the force required to push the O-ring(s) into the spool bore. It is fail-safe due to loss of electrical power to either solenoid, but is not fail safe due to loss of pressure.
U.S. Pat. No. 4,971,115, Four-Way Poppet Valve With Hollow Stem & 4 Port Body, (Tinholt) discloses a 2 position, direct-acting (as opposed to pilot operated), spring offset, balanced spool with fixed elastomeric poppets at each spool land face to provide a zero leakage 4-way, single solenoid operated valve. It is a low pressure pneumatic valve, unsuitable for high pressure operation because the elastomeric poppets would extrude into the gap between the spool and the housing and eventually blow out or get sheared off. Further, the distance betweer the poppet faces and the seat faces need only to be approximately equal because compression of the poppets would make up for small differences and still maintain zero-leakage. If the poppets were metal (for high pressure sealing), the gaps would have to be made and maintained exactly equal. It is not fail safe for either loss of electrical power to either solenoid or loss of "P-port" pressure.
U.S. Pat. No. 5,263,513, Dual Flow Passage Poppet Valve, (Roe) discloses a 2 position, direct-acting, double solenoid, unbalanced multi-poppet design. While the 4 metal-to-metal poppets and seats could classify this as a zero leakage valve, the poppets 146, 148 poppets are sliding fits within the housing 102 and provide two constant leak paths from Pressure to Tank. Should a pair of seals be installed between the poppets and housing, the solenoid force to shift the poppet assemblies would be prohibitive when coupled with the unbalanced poppet design. It is fail-safe due to loss of electrical power to either solenoid, but is not fail safe due to loss of pressure.
U.S. Pat. No. 4,494,572, Four-Way Poppet Valve Assembly, (Loveless) discloses a 2 position, direct-acting, spring offset, lightly unbalanced spools with fixed elastomeric poppets at each spool land face to provide a zero leakage 4-way, single solenoid operated valve. It is a low pressure pneumatic valve, unsuitable for high pressure operation because the elastomeric poppets would extrude into the gap between the spool and the housing and eventually blow out or get sheared off. Should the poppets be made of metal, the unbalanced spool force would have to be increased significantly to maintain its claimed zero-leakage, thus requiring an unacceptable increase in the force (size) of the solenoid and return spring. It is not fail safe for either loss of electrical power to either solenoid or loss of "P-port" pressure.
U.S. Pat. No. 4,791,960, Semi-Pilot Operated Four Way Valve, (Ellison) discloses a 3-way pilot operated poppet valve whose two positions are established by a pressure unbalanced spool, driven by another small direct acting single solenoid, spring offset 3-way valve, which is also used to direct the main flow of the valve to accomplish a 4 way function. It is very flow restrictive because the main flow to and from the second load port must not only pass through the small 3-way pilot valve, but also through a fixed restrictor in the second load port line. The restrictor is necessary to insure sufficient pilot pressure is available to pilot piston 18. This is a low pressure pneumatic valve, unsuitable for high pressure operation because the elastomeric poppets would extrude into the gap between the spool and the housing and eventually blow out or get sheared off. Should the poppets be made of metal, the unbalanced spool force would have to be increased significantly to maintain its claimed zero-leakage, thus requiring an unacceptable increase in the force (size) of the solenoid and return spring. It is not fail safe for either loss of electrical power to either solenoid or loss of "P-port" pressure.
U.S. Pat. No. 4,526,202, Valve With Straight Through Flow, (Chorkey) discloses a single-shoe shear seal design using a flexible conduit fixed at one end (inlet "P" port) to minimize the number of shoes and allow for straight through flow. It is shown to be able to be actuated in several ways: (a) single remote 3-way pilot valve, spring offset, 2 position (piloted from either end); (b) single, direct-acting solenoid, spring offset, 2 position; and (c) double remote 3-way pilot valves, 2 or 3 position. It is a low pressure pneumatic valve, unsuitable for high pressure operation for the following reasons:
(1) the flexible conduit would have to be too rigid (inflexible) to accommodate the high pressure and therefore require a very large force to shift the slider shoe; (2) the high pressure flexible conduit end attachments would have to be too large to be practical; and (3) the pressure loading of the shoe to the slider would have to be substantially increased to minimize leakage, adding further to the need for high forces required to shift the valve. It is not fail safe for loss of power to either the direct acting solenoid or the remote pilot valve(s). Further, it is not fail safe with the loss of "P-port" pressure.
Poppet valve BE4904 manufactured by Wandfluh of America, Inc. is very complex, requiring four 2-way poppet solenoid valves to yield a 4-way function. Pairs of coils must be energized to cause the cylinder to extend or retract. The 4-way Closed Centre Poppet Valve model GGS02-50/51 manufactured by Sterling Hydraulics suffers the same drawbacks.
The Model M-SED 6, Series 1X Directional Poppet Valves with Solenoid Operation manufactured by Mannesmann Rexroth are also very complex. Two 3-way poppet valves are required, with one being solenoid operated and the other being hydraulic pilot operated. The valves are not fail-safe. That is, due to the single solenoid-spring offset operation, if electrical power is lost, pressure to the cylinder is lost. An additional disadvantage is that a relatively high wattage coil is required to hold the poppet on its seat.
The directional seated valves manufactured by Carr Lane Roemheld Mfg. Co. are likewise complex. Two 3-way poppet solenoid valves are required to yield a 4-way function. The valves are not fail-safe due to their dual solenoid-spring offset function. If electrical power is lost, the pressure to the cylinder is lost.